Portable electronic devices may include impedance-matching circuitry. Moreover, antenna/radio performance may be monitored and used to adaptively control a tunable matching circuit or to affect a tunable antenna, in an effort to improve output power under loaded conditions. An example of adaptive matching is using a coupler to sense S-parameters for reflected output power signals for an antenna of a portable electronic device, as well as phase angles between reflected and forward waves, and then using an algorithm to calculate a setting to reduce the reflected power.
Additionally, the use of diversity antennas in portable electronic devices has become common for use in diversity and/or Multiple Input Multiple Output (MIMO) operations in a downlink. Because the diversity antennas may be used exclusively in the downlink, however, they may operate at relatively low power levels, which can increase the difficulty of monitoring diversity antenna performance.